As typically seen in a liquid crystal display device, a thin film transistor formed on a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. Although the field effect mobility of an amorphous silicon thin film transistor is low, it can be formed on a large glass substrate. On the other hand, the field effect mobility of a polycrystalline silicon thin film transistor is high; however, it cannot always be formed on a large glass substrate because of a crystallization process such as laser annealing.
Techniques have attracted attention, where a thin film transistor using an oxide semiconductor is manufactured and applied to electronic devices or optical devices. Examples of the techniques are disclosed in Reference 1 and Reference 2, where a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O based oxide semiconductor used for an oxide semiconductor film and used as a switching element or the like of an image display device.